linux-kernel - Re: [PATCH 3/5] sched: Detect call to schedule from critical entry code

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Date:   Mon, 5 Oct 2020 13:23:53 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Phil Auld <pauld@...hat.com>,
        Marcelo Tosatti <mtosatti@...hat.com>,
        Mel Gorman <mgorman@...e.de>
Subject: Re: [PATCH 3/5] sched: Detect call to schedule from critical entry
 code

On Mon, Oct 05, 2020 at 12:49:17PM +0200, Frederic Weisbecker wrote:
> Detect calls to schedule() between user_enter() and user_exit(). Those
> are symptoms of early entry code that either forgot to protect a call
> to schedule() inside exception_enter()/exception_exit() or, in the case
> of HAVE_CONTEXT_TRACKING_OFFSTACK, enabled interrupts or preemption in
> a wrong spot.
> 
> Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
> Cc: Marcelo Tosatti <mtosatti@...hat.com>
> Cc: Paul E. McKenney <paulmck@...nel.org>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Phil Auld <pauld@...hat.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> ---
>  kernel/sched/core.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 2d95dc3f4644..d31a79e073e3 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -4295,6 +4295,7 @@ static inline void schedule_debug(struct task_struct *prev, bool preempt)
>  		preempt_count_set(PREEMPT_DISABLED);
>  	}
>  	rcu_sleep_check();
> +	WARN_ON_ONCE(ct_state() == CONTEXT_USER);

	SCHED_WARN_ON() ?

No point in unconditionally polluting that path. Although, per MeL, we
should probably invest in CONFIG_SCHED_DEBUG_I_MEANS_IT :/